Method of manufacturing electric discharge tubes



June 26, 1951 J. G. KRONOUER METHOD OF MANUFACTURING ELECTRIC DISCHARGETUBES Filed July 29, 1948 I m w z JOHAN GEORGE KRONOUER A GENT PatentedJune 26, 19 51 METHOD. OF MANUFACTURING ELECTRIC v DISCHARGE TUBES JohanGeorge Kronouer, ,Elndhoven, Netherlands, assignor to Hartford NationalBank and Trust Company, Hartford, Conn., as trustee Application July 29,1948, Serial No. 41,362

In the Netherlands August 19, 1947 The invention relates to a method ofmanufacturing electric discharge tubes, according to which method thecathode has applied to it compounds which are adapted to be dissociatedby heat and thus to produce readily emitting substances, The inventionrelates in addition to electric discharge tubes manufactured by such amethod and to an apparatus for carrying out such a method.

In electric discharge tubes of widely differing kinds, for example radiovalves, gas discharge tubes, X-ray tubes and the like use is frequentlymade of cathodes the emissive substance of which is constituted bycompounds of given elements of the periodic table and is obtainable bydecomposition of other compounds. The most important representatives ofthis kind of cathodes are the alkaline earth oxide cathodes, the oxidesconstituting the emissive substance being preferably obtained asa'result of heating oarbonates or nitrates. A cathode of this kind isgenerally manufactured by applying a supply of the compound to bedecomposed, for example, carbonate or nitrate, to a support, which maybe a coiled filament, for example by immersion in asuspension, thecathode then being mounted the 'tube and heated during manufacture ofthe discharge tube, for example prior to, during or subsequent toexhaustion and expulsion of gases out of the various components of thetube.

It is known that the process of developing readily emitting substancescomprises at least two stages. At the first stage the compounds aredecomposed, a large amount of gas being evolved and compounds, forexample, oxides, being left which at the second stage, owing tosubsequent heating, the so-called,forming operation, exhibit a low workfunction.

The invention is based on recognition of the fact that during theforming operation the atmosphere in the discharge tube must fulfillparticular requirements.

This gaseous atmosphere is formed by gases emanating from varioussources. Firstly there is the gas which is normally contained in thetube, generally therefore air. Secondly, as a result of heating thetube, gas is evolved from all the components thereof, for example thewall and the electrodes. Thirdly, gas is evolved by decomposition of thecompounds on the cathode.

4 Claims. (01. s s-24y Finally there is the possibility of introductionof a so-called rinsing gas into the tube. This will be referred tolater.

It has been found that all these gases do not substantially affect thecathode at the stage of decomposition of the compounds. However, at theforming stage it is necessary that the pressure of the said gases shouldnot exceed a given value, which is low enough to say briefly that duringthe forming operation a complete vacuum should prevail in the tube. Thisis related to the fact that during the forming operation substances(probably metals) are released from the available compounds, for exampleoxides and are liable to react with the gases and to form non-emissivecompounds. This would consequently have the effect of converting part ofthe substances available on the cathode into non-emissive compounds.continue until substantially all the gas would be vinculated or pumpedaway; not until then a cathode of really satisfactory work functioncould be formed. However, in the case of the forming operation beingeffected in a high vacuum, there is only small probability that thedeveloped active substances (metals) of the oathode may combine withgases from the atmosphere in the tube. As a rule, it may therefore besaid that at the beginning of. the cathode forming operation thepressure must be low enough for the development of active substances onthe cathode to exceed the loss of these substances after production dueto the fixing of gas residues from the atmosphere in the tube.

In the methods hitherto adopted the gases available in the tube fromthestart and those evolved from the wall, the electrodes, and so forth,have been allowed for. Exhaustion was therefore continued until a highvacuum prevailed in the tube to be manufactured. The cathode was thenheated. However, it has been insufilciently noted that this heatingoperation had the effect of decomposing the compounds on the cathode,the quantity of gas thus released being such that the pressure in thetube again rises to a value, at which forming is prevented. So far asthis increase in pressure has been ascertained, the exhausting operationwas simply effected for a longer period, so that finally the formingpressure was again reached. As ex- This process would= the gasesreleased from wall, electrodes and. other components of the dischargetube, by the said rinsing gas constitutes a, small improvement but ithas not the effect of preventing evolution .of gases upon decompositionof the compounds on the cathode or of counteracting their harmfuleffect, particularly so, since these gases emanate from the cathodeitself, that is to say precisely at the area at which any activesubstances (metals) already formed areliable to fix them immediately Theaforesaid disadvantages may be greatly reduced by the use of a methodaccording to the invention, which furthermore ensures' additionaladvantages.

In a method according to the invention of manufacturing electricdischarge tubes, according to which the cathode has incorporated in itcompounds which are adapted to decompose by heat into gaseousdecomposition products and substances which, as a result of a subsequentheating operation, the so-called forming operation, in a high vacuum,without any appreciable production of gas, exhibit a low work function,decomposition takes place at an instant of the exhausting and de-gassingprocess to which the discharge tube is subjected, at which the pressurein the tube exceeds the forming pressure, whereupon heating of thecathode is stopped until the forming pressure is reached.

The term forming pressure is to be understood to mean in thisspecification that pressure in the discharge tube, measured subsequentto the decomposition of the compounds available on the cathode, atwhich, as a result of'heating the cathode, the development of substancesof low work function exceeds the loss of these substances due to thefixing of gases from the atmosphere in the tube.

In a method according to the invention, decomposition takes place atsuch an instant of the cycle of manufacturing the discharge tube thatthe gases evolved upon decomposition have disappeared from the tube, atleast in part, be-

As a matter of course, decomposition will be started preferably early inthe cycle so that at the instant at which heating is started, theforming pressure again prevails in the tube, the probability ofproduction of non-emissive compounds being thus slightest.

In addition to this advantage, there is a further material advantage inthat in the use of the standard exhausting pumps, the decrease inpressure per unit time is reduced according as the pressure in thevessel to be exhausted diminishes so that the exhaustion curve exhibitsa substantially exponential variation. Since, according to theinvention, decomposition takes place before the forming pressure andhence a high vacuum is reached the gases evolved upon this-decompositionwill be conducted away considerably more rapidly than in the methodshitherto adopted, in which a high-vacuum was started with. The use ofthe method according 4 to the invention consequently permits of theentire process being materially shortened. Decomposition of thecompounds is brought about preferably at an early stage of theexhausting operation or at least before the pressure is reduced to lessthan 20 times the forming pressure. The cathode may be heated. forexample, practically immediately at the start of the exhaustion, sinceat this instant the decrease in pressure per unit time is considerableand it has been found that the gases evolved from the other componentsof the discharge tube, for example water vapour, practically do notaffect the com-' pounds thus formed on the cathode.

After the compounds on the cathode are decomposed the cathode is notheated until the forming pressure has been reached, since heating, whencontinued, has the effect of produc-. ing non-emissive compounds bywhich the cathode is rendered brittle.

When heating the cathode is referred to, this is to be understood tomean the particular heating of the cathode.

In the method according to the invention use may be made of a rinsinggas, for example argon. However, compared with the methods hithertoadopted, this does not offer my particular advantages.

Upon the cathode being formed, the discharge tube is treated in theusual manner in the method according to the invention.

Owing to the differences in construction between the various kinds ofdischarge tubes to which the invention may be applied, the effect willnot always be identical, that is to say the gain of time achieved willnot always be a like part of the total time involved for the manufactureof the discharge tube. With radio valves, for example, the capacity ofthe vessel is small, but the number of components to be de-gassed isgreat. With gas discharge tubes, operating at low pressures, thecapacity is material, whereas the number of components to be de-gassedis comparatively small. These components, however, are to be understoodto include the tube wall, and, if any, the luminescent layer applied toit, whichis particularly large.

In order that the invention may be more clearly understood and readilycarried into effect, it will now be described more fully with referenceto the accompanying drawings.

Figure 1 shows a graph of the pumping time for tube evacuation plottedagainst the pressure in the tube, and Fig. 2 illustrates the sequence ofoperations of the apparatus used in the method of invention.

In the graph shown in Fig. 1 the pumping time T is plotted on thehorizontal axis, the pressure P on the vertical axis. With a givendischarge tube and a given pump the exhaustion curve which would occurif the cathode were not heated is designated by the curve a. This curvehas a substantially exponential variation but for two kinking points.The kinking point A is due to the heating and de-gassing operations towhich the tube and its components are subjected being started; thekinking points on the curve L-l are produced upon exit of the tube fromthe heating zone (furnace). The dotted line parallel to the horizontalaxis designates the forming pressure. Curve a intersects this dottedline at point Q and according to the methods hitherto adopted, heatingof the cathode has invariably been started at this point. Owing to thisheating a quantity of gas is evolved, so that the pressure evolved issubstantially equal.

creases to the value V. From this instant the pressure decreases asshown by curve c. How:

ever, the decrease in pressure in the zone between U and Q is quitematerial, since the pumping rate is high. The curve intersects the lineof the forming pressure at a point W. The use of the method according tothe invention therefore results in a gain in time corresponding to thedistance W-S.

In order that the invention may be clearly understood an apparatus willbe briefly described which permits of carrying out the method accord-The pressure ining to the invention. This apparatus will be described asconvenient for the exhaustion of lowpressure mercury-vapour dischargelamps but without departing from the scope of the invention, it may bearranged to be convenient for the manufacture of other kinds ofdischarge tubes.

The apparatus comprises a so-called rotary pump system having 36positions as shown in Fig. 2. In each of these positions the gas discharge tube may be subjected to a definite treatment. Positions I, 2 and3 are required, for example, for the insertion of the tube. During therotation of the pump system the tube is connected in the next followingposition 4 to a vacuum conduit and remains connected thereto as far asposition 30, so that the pressure in the tube is reduced. Next, the tubepasses through a number of positions, for example, I to 18, which arelocated in a furnace in which heating of the tube and its parts iseffected. After this furnace a number of positions, for example I! to30, are available for firing the cathode and after this firing a furthernumber of positions, for example 30 to 35, are available for theintroduction into the tube of a loading gas, for example argon, and ofthe mercury required for the discharge. In the method hitherto knownheating of the cathode was started after the tube has passed through thefurnace, that is to say, in position l9. In the apparatus for carryingout the method according to the invention firing is effected in position5, i of the cycle. The switches required for this purpose are providedon the apparatus.

What I claim is:

1. The method of manufacturing an electric discharge tube having anenvelope and within the said envelope a cathode having a surface ofemissive substances of low work function formed from a heat-decomposablecompound applied to the said cathode, comprising the steps of evacuatingthe gaseous filling of the said envelope to a given pressure greaterthan the pressure at which the development of the said emissivesubstances of low work function exceeds the loss of the developedemissive substances occurring from the interaction of the said emissivesubstances with the gases of the atmosphere of the tube, heating thesaid cathode at the said given pressure to decompose the said compoundand increase the pressure within the said envelope, further evacuatingthe said envelope to a pressure at which the development of emissivesubstances of low work function exceeds the loss of the developedemissive substances occurring from the interaction of the emissivesubstances with the gas of the tube atmosphere, and thereafteractivating the said cathode surface to produce the emissive substancesof low work functionthereon while continuing the evacuation of the saidenvelope.

2. The method of manufacturing an electric discharge tube having anenvelope and within the said envelope a cathode having a surface ofemissive substances of low work function formed from a heat-decomposablecompound applied to the said cathode, comprising the steps of evacuatingthe gaseous filling of the said envelope to a given pressure which isapproximately twenty times the pressure at which the development of thesaid emissive substances of low work function exceeds the loss of thedeveloped emissive substances occurring from the interaction of the saidemissive substances with the gases of the atmosphere of the tube,heating the said cathode at the said given pressure to decompose thesaid compound and increase the pressure within the said envelope,further evacuating the said envelope to a pressure at which thedevelopment of emissive substances of low work function exceeds the lossof the developed emissive substances occurring from the interaction ofthe emissive substances" with the gas of the tube atmosphere, andthereafter activating the said cathode surface to produce the emissivesubstances of low work function thereon while continuing the evacuationof the said envelope.

3. The method of manufacturing an electric discharge tube having anenvelope and within the said envelope a cathode having a surface ofemissive substances of low work function formed from a heat-decomposablealkaline earth carbonate applied to the said cathode, comprising thesteps of evacuating the gaseous filling of the said envelope to a givenpressure greater than the pressure at which the development of the saidemissive substances of low work function exceeds the loss of thedeveloped emissive substances occurring from the interaction of the saidemissive substances with the gases of the atmosphere of the tube,heating the said cathodeat the said given pressure to decompose the saidalkaline earth carbonate and increase the pressure within the saidenvelope, further evacuating the said envelope to a pressure at whichthe development of emissive substances of low work function exceeds theloss of the developed emissive substances occurring from the interactionof the emissive substances with the gas of the tube atmosphere, andthereafter activating the said cathode surface to produce the emissivesubstances of low work function thereon while continuing the evacuationof the said envelope.

4. The method of manufacturing an electric discharge tube having anenvelope and within the said envelope a cathode having a surface ofemissive substances of low work function formed from a heat-decomposablealkaline earth nitrate applied to the said cathode, comprising the stepsof evacuating the gaseous filling of the said envelope to a givenpressure greater than the pressure at which the development of the saidemissive substances of low work function exceeds the loss of thedeveloped emissive substances occurring from the interaction of the saidemissive substances with the gases of the atmosphere of the tube,heating the said cathode at the said given pressure to decompose thesaid alkaline earth nitrate and increase the pressure within the saidenvelope, further evacuating the said envelope to a pressure at whichthe. development of emissive substances of low work function exceedsthe-loss of the developed emissive substances occurring from theinteraction of the emissive substances with the gas of the tubeatmosphere, and thereafter activating the said cathode surface toproduce the emissive substances of low work function thereon whilecontinuing the exacuation of the said envelope.

JOHAN GEORGE KRONOUER.

Qua-1.91

8 .HREFERENCES v01mm The following references'are of record in the fileof this patent:

UNITED STATES PATENTS Number Name Date Espe et'al. Jan. 24, 1933

